Stem cells such as embryonic stem cells (ES cells) and induced pluripotent stem cells (iPS cells) derived from humans have an ability to differentiate into a wide variety of cell types. By mass culturing and then differentiating these stem cells into the cells of interest, they can be applied to conventionally difficult large-scale drug efficacy evaluation or medical practice using human cells such as elucidation of disease, drug discovery screening, toxicity test, or regenerative medicine, and in light of this, stem cells are attracting attention.
It is to be noted that, in the process of culturing stem cells while keeping them in the undifferentiated state, some cells fail in maintaining the undifferentiated state and go into the state tending toward differentiation, and from time to time the emergence of such cells led to a deterioration of the quality of the whole stem cells. In light of this, the importance of quality control of stem cells is pointed out; however, in order to exercise proper quality control, it is necessary to monitor stem cells, determine whether the cells are in the differentiated or undifferentiated state, and remove the cells in the state tending toward differentiation, which no longer maintain the undifferentiated state.
As a method for determining whether the cells are in the differentiated or undifferentiated state as mentioned above, conventionally, using an optical microscope such as a phase contrast microscope, a stem cell colony in the undifferentiated state is distinguished from a stem cell colony in the state tending toward differentiation by visually determining transmitted light images obtained by transillumination (for example, see page 17 of Non Patent Literature 1). Then, the colony that is determined to be in the state tending toward differentiation by visual determination is eventually collected and then removed by aspiration using a pipette, etc.